Method of coating



2,913,385 METHOD OF COATING Daniel Satriana, Verona, and George Rah,Pine Brook,

N.J., assignors to Vitro Corporation of America, Verona, NJ.

No Drawing. Application May 28, 1953 Serial No. 738,291 8 Claims. (Cl.204181) This invention relates to the preparation of coatings on thesurface of a substrate.

The present invention is an improvement over the process disclosed inthe copending application Serial No. 656,315, filed May 1, 1957, ofHarold N. Barr, now United States Patent No. 2,878,140, issued March 17,1959. The said application discloses and claims a process for placingpore free coatings on a substrate. The process involves the necessity ofplacing the substrate coated with sinterable particles in a soft rubberor plastic bag or sleeve which is then evacuated and drawn into contactwith the coated substrate. The encased substrate is then isostaticallypressed in a chamber filled with a liquid to densify the coating ofsinterable particles prior to sintering.

Such densification procedure has been found to be desirable inpreventing any tendency of the primary coating to slough off or to bedamaged through mishandling. The need for densification of the coatingis particularly apparent in the formation and treatment of coatingsprepared by electrophoresis in accordance with a particular embodimentof the invention as described hereinafter.

One of the difficulties experienced with the above process is that incoating substrates having apertures therethrough, or having a complex orhighly irregular surface the bag or sleeve cannot be satisfactorilydrawn into contact with the coated substrate so that they conform to theirregular surface of the coated substrate. When pressed, the coatingtherefore is not uniformly and evenly densified over the surface area ofthe substrate. A further difiiculty encountered is that the bagging andsubsequent steps necessary for isostatic pressing are somewhatintricate, tedious and time consuming.

Accordingly, it is an object of the present invention to eliminate thedifliculties encountered in the prior process by permitting thedensification of coatings on substrates which cannot be handled by thebagging technique.

It is also an object of the present invention to eliminate the costlyand intricate steps of bagging and applying vacuum in the process ofdensifying coatings.

It is a further object to provide a new process for densifying coatingson regular or irregularly shaped objects.

A further object is to reduce the cost of densifying coatings to makethe process more attractive economically.

It has been found that by applying a thin wax covering on the coatedsubstrate the substrate can then be isostatically pressed using 20 to 60tons per square inch pressure to densify the coating without thenecessity of employing the bagging technique required in the priorprocess. The amount of pressure exerted is governed by the compositionof the sinterable particle coating. While the densification can beaccomplished solely by means of a wax covering, it is preferred first tocover the coated substate with a liquefied film-forming polymer whichwill form a film on the covered substrate and then to apply the wax tothe polymer film. This new process eliminates the bagging technique andits inherant disadvantages.

The wax covering should be thin, in the range of 2 to 19 mils(preferably 5 to 15 mils), free of air bubbles and pit holes and shouldcompletely cover the coated substrate. It has been found that thickerwax coatings, 20 to 30 mils in thickness, may crack during isostaticpressing. The substrate, thus coated, is then placed in an isostaticpress to densify the coating and thereafter heated to sinter thesintering particles on the substrate. If desired the substrate can beplaced in a protective preformed pliable container, such as a rubber orplastic bag, filled with glycerine and then isostatically pressed inorder to prevent marriug of the substrate by the pressure device.

Coatings may be formed on a variety of substrates such as metals,graphite and ceramic. Moreover, a variety of coating materials may beused, including ceramics, metals and metal oxides. However, the choiceof coating material for the particular substrate should be such that thesinterable ingredient of the coating may be sintered at a temperaturewhich is substantially below the melting point of the substrate.

The preferred procedure is first to cover the sinterable coating on thesubstrate with a liquefied film-forming polymer which will form a filmon the coated substrate. The thin wax covering is then applied, and thesubstrate is then pressed to densify the coating and heated to sinterthe sinterable particles on the substrate. The heat applied to sinter issufiicient to remove the Wax and polymer film coatings before thesintering temperature is reached. These coatings can also be removed bysuitable solvents or by stripping them off the substrate.

As a first step in preparing the metal substrate for treatment, thesinterable particles whether they be metal, metal oxide or ceramic ormixtures of any two or more of these types of materials, may be appliedto the surface of the metal substrate by a variety of procedures such aselectrophoretic deposition, spraying, dipping or painting. However, itis preferred to use electrophoretic deposition. In the electrophoreticcoating process, a suspension of the sinterable particles to bedeposited in a suitable liquid vehicle (preferably an organic solvent)is prepared. The suspension may be prepared by ball milling thesinterable material in the solvent, for example alcohol, to obtain afinely dispersed material. Particles ranging in diameter from 0.5 to 10microns are preferred, although dispersions containing particles from0.1 to 74 microns have been prepared and deposited. The electricalcharge on the particle is acquired during the dispersing operationbecause of absorption of ionizable substances or a reaction between thesolid and liquid. The organic liquids that are preferred for use assuspending vehicles prevent electrolytic reaction and the gassing of theelectrodes. The article to be coated is made one of the electrodes in abath of the suspension. The suspension is preferably agitated slowlyduring deposition to prevent settling. Cell voltages of 200 to 1000volts DC. are permissible because the vehicle is an anhydrous solvent.The surface of the metal substrate is preferably thoroughly cleanedbefore it is immersed in the plating bath. Coating thicknesses can bevaried with the deposition time, electrode spacing, voltage andsuspension concentration.

Any film-forming polymer may be used which will decompose, vaporize oreven oxidize (provided the oxidizing conditions will not irreparablyharm the coating) before the sintering temperatures are reached leavingno deposit to poison the sinterable coating. Examples of suitablefilm-forming polymers are acrylic resins, polyvinyl chloride, polyvinylacetate, polyvinyl alcohol, polystyrene, cellulose acetate, etc. Thecovering of filmforming polymer may be applied in a liquefied form byspraying, dipping or brushing solutions, plastisols, or otherdispersions, or even by applying powdered polymer to the heatedsubstrate thereby to fuse the powder. A preferred polymer is Krylon, acommercial acrylic spray which is composed of acrylic resin, 15%chlorinated solvents, 30% aromatic hydrocarbon and 50% propellant(fluorinated hydrocarbons).

Suitable waxes are mineral, semi-mineral, animal, insect, vegetable andsynthetic waxes, such as ceresine, beeswax, carnauba Wax, japan wax,montan wax, spermaceti and Carbowax 4,000. Here again any wax may beused which will decompose, vaporize or oxidize before the sinteringtemperatures are reached leaving no deposit to poison the sinteringparticle coating and which will deform without breaking at pressuresranging from 15 up to 60 tons per square inch. The Wax is preferably.applied by dipping the coated substrate into molten wax. Other methodssuch as spraying hot wax over the cool substrate can also be employed.

The following examples are presented as illustrations of the invention.

Example 1 A wedge of molybdenum alloy having deep holes piercing thewedge was cleaned by the standard procedure of degreasing, and was acidetched, and lightly sandblasted to roughen the surface. A dispersion wasthen prepared containing 104 g. nickelous oxide, 20 g. chro mium powder,and 1 g. ethyl cellulose in 500 ml. of Z-nitropropane. This dispersionwas then used to coat the molybdenum wedge electrophoretically. At 500volts DC. and 12 milliamperes current, a coating equivalent toapproximately 2 mils of Nichrome was deposited in 15 seconds. Thecoating was reduced at 400 C. for onehalf hour in purified hydrogen. Thecoated molybdenum substrate was then dipped into molten ceresine, keptat a temperature from 90 to 100 C., to form a thin coating having athickness of about mils on the substrate. The substrate was slightlyagitated when submerged in the hot ceresine to insure that all the airbubbles were expelled and that complete coverage of the substrate wasobtained. The thus coated substrate was removed from the hot ceresineand allowed to cool in order to harden the wax coating.

The wax covered molybdenum wedge was then placed in the isostatic pressand pressed hydraulically at 25 tons per square inch. After removal fromthe press, the molybdenum wedge was heated at about 1100 C. in anatmosphere of hydrogen for about 20 minutes to sinter the coating. Testsshowed high density and good bonding of the coating to the molybdenumbase.

Example 2 The same process as employed in Example 1 was carried out withthe exception that the molybdenum Wedge coated with a ceresine layer wasplaced in a rubber bag filled with glycerin and isostatically pressed.Here again the coating was dense, well sintered and firmly bonded to themolybdenum substrate, and had a low porosity.

Example 3 The same process as employed in Example 1 was carried out withthe exception that the substrate coated with the sinterable layer wasfirst sprayed with Krylon, a commercial acrylic resin polymer spray, toform a thin film of polymer on the surface of the coated substrate andthen dried. The coated and polymer film covered molybdenum substrate wasthen dipped into molten ceresine, kept at a temperature of from 90 to100 C., to form a thin wax covering on the substrate. The substrate wasthen further processed as set forth in Example 1.

In place of the metal powders used in Examples 1 to 3, nickel oxide,nickel, columbium carbide, chromium, iron oxide, molybdenum, andmixtures of these powders deposited on the surface of the substrate byelectro- '4 phoresis, electroplating, spraying, dipping or brushing maybe employed as the sinterable particle coating. In all instances, highdensity coatings with good bonding to the metal base are obtained.

The coatings formed by the process of this invention may have densitieswhich are about -99% of the theoretical or intrinsic density of thecoating material, which indicates a very low porosity.

The novel process of the present invention greatly simplifies theprocess of densification of coatings by eliminating the time consuming,tedious and costly bagging step. Further, the present process permitscoatings of good quality on irregularly shaped and cast objects whichcannot be satisfactorily coated by employing the bagging technique fordensification.

It is to be understood, as noted above, that changes and variations maybe made without departing from the spirit and scope of the inventiondefined in the appended claims.

We claim:

1. In the preparation of a coating on the surface of a substrate thesteps of coating said surface with sinterable particles, covering saidcoated surface with liquefied wax, solidifying said wax covering,applying isostatic hydraulic pressure to the wax and to the coatingwhich the wax covers thereby to densify the coating, and thereaftersintering said coating on said substrate.

2. The process as set forth in claim 1, wherein the surface of thesubstrate is coated by electrophoresis.

3. The process as set forth in claim 1, wherein the wax covering isapplied by dipping the coated substrate into molten wax.

4. The process as set forth in claim 2, wherein the thickness of the waxcovering is in the range of 5 to 15 mils.

5. The process of claim 1 wherein the wax covered and coated substrateis placed in a preformed pliable container filled with glycerin and thenthe isostatic pressure is applied thereto.

6. In the preparation of a coating on the surface of a substrate, thesteps of coating said surface with sinterable particles, applying aliquefied film-forming polymer on said coated surface, solidifying saidpolymer to form a film on said coated surface, covering said coated andfilmed surface with liquefied wax, solidifying said wax covering,applying isostatic hydraulic pressure to the wax and to the coatingwhich the wax covers thereby to density the coating, and thereaftersintering said coating on said substrate.

7. The process as set forth in claim 6, wherein the liquefiedfilm-forming polymer is sprayed onto the coated surface and the Waxcovering is applied by dipping the coated substrate into molten Wax.

8. In the preparation of a coating on the surface of a metal, the stepsof coating said surface with sinterable particles, spraying liquefiedacrylic resin on said coated surface, drying said resin to form a filmon said coated surface, covering said coated and filmed surface withabout a 10 mil thickness of molten ceresine, cooling the substrate toharden the ceresine covering, applying isostatic hydraulic pressure tothe ceresine and to the coating which the ceresine covers thereby todensify the coating, and thereafter sintering said coating on saidsubstrate.

References Cited in the file of this patent UNITED STATES PATENTS1,819,336 Penning Aug. 18, 1931 2,386,634 Robinson Oct. 9, 19452,389,060 Kurtz Nov. 13, 1945 2,421,652 Robinson et al. June 3, 19472,671,955 Grubel et al Mar. 16, 1954 OTHER REFERENCES Shyne et al. inPlating, October 1955, pages 1255- 1258.

1. IN THE PREPARATION OF A COATING ON THE SURFACE OF A SUBSTRATE THESTEPS OF COATING SAID SURFACE WITH SINTERABLE PARTICLES, CONVERING SAIDCOATED SURFACE WTH LIQUEFIEXD WAX, SOLIDIFYING SAID WAX COVERING,APPLYING ISOSTATIC HYDRAULIC PRESSURE TO THE WAX AND TO THE COASTINGWHICH THE WAX COVERS THEREBY TO DENSIFY THE COATING, AND THEREAFTERSINTERING SAID COATING ON SAID SUBSTRATE.